p24/CD9 is an integral membrane protein found on the surface of human blood platelets, lymphoid progenitor cells, mitogen-stimulated T and B cells, endothelial cells and other non-hematologic or vascular cells. The broad yet selective expression of the protein has suggested a fundamental role in cellular function. The applicant has acquired evidence that p24/CD9 functions in cellular adhesion. She has demonstrated that: 1) fibronectin is a ligand for affinity-purified native CD9 as well as recombinant CD9; 2) CD9 transfected CHO cells spread to a greater extent on fibronectin coated surfaces; 3) CD9 transfected CHO cells have greater mobility on fibronectin coated filters; 4) CD9 mediates platelet adhesion to fibronectin, and 5) adhesion can be inhibited by anti-CD9 monoclonal antibodies and synthetic peptides corresponding to external CD9 sequences. The long term goal of the proposed is to further define CD9-ligand interactions using a purified system and to characterize how CD9 mediates cellular adhesion and motility. In the first specific aim, she will characterize the fibronectin binding properties of CD9 and identify any other potential ligands as well as define functional domains on CD9 that mediate binding. She will also determine whether p24/CD9 interacts physically or has any functional ties with the b1 integrins and will explore the preliminary finding that CD9 interacts with GpIIb/IIIa. The second specific aim will define the mechanisms of CD9 mediated cell interactions with fibronectin and other ligands and determine the molecular requirements for these interactions. The model systems will include CHO cells expressing CD9 developed in the applicant's laboratory, and platelets. The hope is that these studies will establish the adhesive properties of p24/CD9 and clarify its role in platelet and other cell function.